Antibody molecules consist of a combination of two heavy (H) chain and two light (L) chain polypeptides. Each heavy and light chain comprises a constant region containing the CL, CH1, hinge region, CH2, and CH3 regions, and a variable region containing the hypervariable regions (complement determining regions (CDRs)); the CDRs control the antibody's antigen-binding characteristics. The two heavy chains are joined to each other and the light chains in a Y-shaped structure via disulfide bridges such that the variable regions of the light chains (V.sub.L) and heavy chains (V.sub.H) are located next to each other.
To generate antibodies, conventional hybridoma techniques have been used in which clones of hybrid cells expressing genes coding for the light and heavy chains of an antibody molecule are obtained by immunization with an antigen molecule. This technique necessitates the fusion of cells of lymphocytic origin, containing the genes for antibody formation and cells forming immortal lines. The cells carrying the genes in question are generally obtained by random creation of libraries of circulating cells, and screening of the hybridomas with an antigen-antibody reaction after the hybridoma clones are multiplied and cultured. This technique can be uncertain and laborious with limited yield of antibodies, and is limited in application to non-human (e.g., mouse) antibody production.
In addition, monoclonal antibodies and their fragmants can be expressed in various host systems, such as E. coli, yeast, and mammalian host cells. In general, a mammalian expression vector will contain (1) regulatory elements, usually in the form of viral promoter or enhancer sequences and characterized by a broad host and tissue range; (2) a “polylinker” sequence, facilitating the insertion of a DNA fragment within the plasmid vector; and (3) the sequences responsible for intron splicing and polyadenylation of mRNA transcripts. This contiguous region of the promoter-polylinker-polyadenylation site is commonly referred to as the transcription unit. The vector will likely also contain (4) a selectable marker gene(s) (e.g., the.beta.-lactamase gene), often conferring resistance to an antibiotic (such as ampicillin), allowing selection of initial positive transformants in E. coli; and (5) sequences facilitating the replication of the vector in both bacterial and mammalian hosts.
Unlike most genes that are transcribed from continuous genomic DNA sequences, antibody genes are assembled from gene segments that may be widely separated in the germ line. In particular, heavy chain genes are formed by recombination of three genomic segments encoding the variable (V), diversity (D) and joining (J)/constant (C) regions of the antibody. Functional light chain genes are formed by joining two gene segments; one encodes the V region and the other encodes the J/C region. Both the heavy chain and.kappa. light chain loci contain many V gene segments (estimates vary between 100 s and 1000 s) estimated to span well over 1000 kb. The.lambda. locus is, by contrast, much smaller and has been shown to span approximately 300 kb on chromosome 16 in the mouse. It consists of four joining/constant region gene segments and two variable gene segments. Recombination resulting in functional genes occurs predominantly between V.sub.1 and either J.sub.1/C.sub.1 or J.sub.3/C.sub.3 elements or between V.sub.2 and J.sub.2/C.sub.2 elements (J.sub.4/C.sub.4 is a pseudogene), although recombinations between V.sub.2 and J.sub.3/C.sub.3 or J.sub.1/C.sub.1 are seen very rarely.
An example of a mammalian expression vector is CDM8. The transcription unit of CDM8 is composed of a chimeric promoter (the human cytomegalovirus AD169 constitutive promoter fused to the T7 RNA polymerase promoter), a polylinker region and the SV40 small tumor (t) antigen splice and early region polyadenylation signals derived from pSV2. The human cytomegalovirus (HCMV) promoter is expressed in a variety of mammalian cell types, while the T7 bacteriophage DNA-dependent RNA polymerase promoter can drive in vitro cell-free transcription/translation of cloned inserts. This particular promoter fusion allows initial experiments to be conducted within the confines of the host mammalian cell type, while further analysis and utilization of the cloned insert may potentially be carried out in an in vitro “cell-free” transcription/translation system. The constitutively expressed HCMV promoter has also been utilized in other mammalian expression vectors besides CDM8. Origins of replication in CDM8 include (1).pi.VX (allowing e.g., replication in E. coli) (2) SV40 origin (e.g., allowing replication in a variety of COS cell types) (3) polyoma origin (e.g., allowing replication in polyoma virus transformed mouse fibroblasts) and (4) the bacteriophage M13 origin (e.g., allowing generation of single-stranded template for DNA sequence analysis and/or oligonucleotide site-directed mutagenesis).
Furthermore, CDM8 carries the supF gene for selection in E. coli. In this antibiotic selection system, a CDM8-based plasmid construction is transformed into a specialized E. coli strain containing an episome carrying genes encoding resistance to the antibiotics, ampicillin and tetracyline. However, both genes contain chain termination (“nonsense” codon) point mutations inactivating the resistance phenotype. The supF gene product, a nonsense suppressor tRNA, restores the resistant phenotype for each antibiotic. Therefore, selection is based on growth of the specialized episomal-carrying E. coli strain on media containing ampicillin and tetracycline. Colonies exhibiting this phenotype are supposedly transformed with the CDM8-based plasmid construction.
The CDM8 vector is compatible with COS cell lines as well as cell lines transformed with the polyoma virus. COS cell lines are African green monkey CV1 cells transformed with an origin-defective SV40 mutant virus. The COS cells produce the large T antigen, which is required in trans to promote replication of SV40 or plasmid constructions, such as CDM8, which contain the respective cis-acting sequences initiating viral replication. Therefore, COS cells transfected with a CDM8-based construction will support replication of the plasmid, resulting in increased plasmid copy number and a transient overexpression of the gene of interest.
The major use of CDM8 is cDNA expression cloning and overproduction of specific proteins in a mammalian in vitro expression system. Expression cloning takes on various forms depending on the mode of detection utilized to identify the cDNA of interest; however, the initial step consists of isolating mRNA and synthesizing double-stranded deoxyribonucleic acid copies of the mRNA population (cDNAs). These cDNAs must be efficiently ligated to a plasmid or bacteriophage DNA cloning vector and transferred to the appropriate host prior to library screening and analysis. The CDM8 vector contains two BstXI restriction sites, making it amenable to the “adaptor” linker procedure of ligating cDNAs to the vector, i.e., the use of DNA fragments blunt ended at one end (and therefore compatible for ligation with the blunt ended cDNA) but containing a non-palindromic overhang (sticky end) on the other end (in this instance, compatible for ligation with BstXI digested vector DNA, but not with other cDNAs).
Another example of a mammalian expression vector is pCMX. This vector contains (1) the immediate early promoter of HCMV, (2) an SV40 RNA splice/polyadenylation sequence, (3) an SV40 origin of replication, (4) a pBR322 origin of replication and (5) a selectable marker conferring resistance to an antibiotic, such as the.beta.-lactamase gene conferring resistance to the antibiotic ampicillin. The pCMX vector can also be used for the transient expression of a cloned DNA sequence in transfected COS cells. Control of transcription of both rearranged heavy and.kappa. light chain genes depends both on the activity of a tissue specific promoter upstream of the V region and a tissue specific enhancer located in the J-C intron. These elements act synergistically. Also, a second B-cell specific enhancer has been identified in the.kappa. light chain locus. This further enhancer is located 9 kb downstream of C.sub. kappa.
One such mammalian host system used to produce antibodies is a mouse myeloma host cell that has been transfected with cloned DNA encoding the desired antibody. Such “recombinant monoclonal antibodies” are often distinct from hybridoma-derived monoclonal antibodies for which the DNA has not been cloned and for which the cells producing the monoclonal antibody are derived by immortalizing a natural monoclonal antibody-producing cell isolated from an animal. The heavy and light chain immunoglobulin (Ig) genes being expressed in hybridoma cells are under the control of the natural endogenous promoter that had always been linked to the particular variable region sequence being expressed as opposed to the promoter contained in the recombinant vector.
In recombinant production, the monoclonal antibody sequence to be cloned must be ligated into an appropriate vector after restriction enzyme treatment of the vector. This task can be difficult and imprecise as the process of incorporating the antibody nucleotide sequence(s) into an expression vector or plasmid is complex.
However, by cloning the monoclonal antibody DNA sequences prior to preparing transfected cell-derived monoclonal antibodies, recombinant DNA methods can be used to replace the natural endogenous promoter for an Ig gene with any promoter of choice. A primary reason for changing a promoter is to realize higher monoclonal antibody production levels.
Promoter sequences, in conjunction with downstream enhancer sequences, are responsible for driving transcription (i.e., RNA synthesis) of the heavy and light chain genes in the transfected cells by binding to specialized nuclear proteins called transcription factors. It has become apparent that there are fewer sites for transcription factor binding in an Ig promoter than there are in an Ig enhancer; however, the fact that there is sequence variability among promoters but only a single copy of an enhancer sequence makes it highly likely that there is functional variability among Ig promoters. One promoter may be “strong,” i.e., efficient at binding a favorable combination of transcription factors that leads to high levels of monoclonal antibody RNA synthesis, whereas another promoter may be “weak,” due to having a different DNA sequence. Since each of the more than 200 variable region HC genes and the more than 200 variable region LC genes in an Ig repertoire has its own naturally linked promoter, and it is likely that no two promoters have identical sequences, the many different Ig promoters are likely to vary significantly with respect to how well they drive transcription.
Ig promoters are only functional in lymphoid-type host cells, such as T cells and B cells (and myeloma cells), due to their requirement for Ig gene-specific transcription factors (for example, Oct-2 and OBF-1) not expressed in other cell types. In addition, even lymphoid cell-specific transcription factors may be expressed only at particular stages of cellular differentiation such that optimal expression may depend on matching the differentiation state of the host cell line with the appropriate sequence motifs in the Ig gene promoters. Although the host cell specificity of Ig promoters may be seen as a minor disadvantage for expression of the monoclonal antibody in a non-lymphoid host cell, the large assortment of HC and LC promoters affords a chance to identify and perhaps further optimize strong promoters that can be incorporated into lymphoid cell-specific vectors.
Expression of monoclonal antibodies behind a strong promoter increases the chances of identifying high-producing cell lines and obtaining higher yields of monoclonal antibodies. Consequently, Ig vectors with strong promoters are highly desirable for expressing any monoclonal antibody of interest. In addition, vectors with unique DNA cloning sites downstream of strong promoters would have an added convenience.
Accordingly, there is a need for new vectors and plasmids useful for expression of antibodies that simplify ligation techniques and enable customization of enhancer and promoter sequences in order to increase antibody production.